1. Field of the Invention
The present invention relates to phase shifters and particularly to a phase shifter for shifting the phase of directly sampled bandpass signals without having to convert to complex (I,Q) form.
2. Description of Related Art
In direct sampling phase shift processing, a sequence of values, r.sub.n, is sampled directly from a signal r(t), assuming that all requirements of direct sampling are met. Consider r.sub.n ' to be direct samples from r' (t), where r' is identical to r, except for a phase shift .phi..
To shift the phase of a directly sampled signal sequence, an appropriate alteration of the sequence must be performed so that the new sequence approximates direct samples from the phase shifted signal.
Several known direct sampling phase shift processing (SOA) techniques for obtaining r.sub.n ' from r.sub.n are discussed below.
1) A first technique for obtaining r.sub.n ' from r.sub.n is to utilize an analog phase shifter before an analog-to-digit (A/D) converter. The analog phase shifter provides r' (t) from r(t). Thus, values sampled by the A/D converter from r' (t) are the r.sub.n ' by definition. PA1 2) A second technique for obtaining r.sub.n ' from r.sub.n would be to sample In and Qn from the in-phase, I(t), and quadrature, Q(t), components of the signal r(t), respectively, and to compute the r.sub.n ' from the r.sub.n by complex multiplication. PA1 3) A third technique for obtaining r.sub.n' from r.sub.n would involve computing In and Qn from the r.sub.n according to a process described in U.S. Pat. No. 4,468,794 and again performing the phase shift computation by complex multiplication.
This first technique requires an analog phase shifter and an additional A/D converter in a separate channel to provide r.sub.n '. Analog phase shifters require biased ferrites, switched transmission lines, or heterodyning with a phase shifted local oscillator. Each one of these additional components is considerably larger, more costly, and less accurate than applicant's invention described in this application.
This second technique requires I and Q demodulators and A/D converters for each demodulator. It also requires the necessary storage, multiplications, and addition required to compute r.sub.n ' from In, Qn and .phi.. Applicant's invention requires no I and Q demodulators (and hence is fundamentally more accurate), only one A/D converter, and about the same digital hardware.
In the third technique, the In and Qn values computed from r.sub.n values require a (2k+1)-stage shift register, k multiplications, k additions, and 2 single-pole, 4-throw digital switches in addition to circuits for computing the r.sub.n ' from the In, Qn and .phi.. This is more accurate than the second technique, but requires about twice as much digital hardware to obtain In and Qn as that required to compute r.sub.n ' directly from r.sub.n using applicant's invention.